I am having persistent cold over the past few months. Please suggest me a medicine.?

See doctor. Cough can be caused by multiple causes. Cough suppressants just cover the symptom but you need to find out the underlying cause with the help of a medical professional.
MD visit. People in your situation need to see their primary md or an ent!it could very well be chronic sinusitis or even new onset allergies (hard to tell with brief details). I tell people that they can can try salt water flushes (believe it or not - it works well - neilmed sinus rinse is one - otc). The other otc med is Mucinex (guaifenesin) (which helps loosen the phleghm and works well with the salt water flush).